Abstract
1. An increase in apparent renal clearances is frequently observed on restoring urine flow after a period of anuria or on increasing it after oliguria. An analysis of such 'peaks' in clearance has been made in experiments on anaesthetized dogs, using two preparations of labelled vitamin B12 and urine collections of 1-2 min. [57Co]B12 was infused throughout the experiments, while [58Co]B12 was given as a single injection during periods of anuria or oliguria induced by noradrenaline infusion, haemorrhage or aortic obstruction. 2. The apparent high clearance in the first minute or two of restored or increased flow is an artifact explained by inclusion in the peak of material filtered earlier, but not excreted. By means of the integrated plasma concentration ratio of the two B12 isotopes during the period of low or absent flow, the excess B12 in the peak may be reapportioned between the period before the 58Co was injected and the period after it. 3. The findings indicate that filtration may temporarily continue during anuria, but this is concealed as a result of failure of onward flow of filtrate. In oliguria a similar concealment of filtration may result from the cessation of onward flow in some nephrons.